Switch mode power supplies have been widely used in a great variety of applications and appliances which require light and compact regulated power sources of high efficiency. In addition, said power supplies have high reliability and low power loss, and they can easily be configured to step up and down supply voltages in accordance with design requirements.
In a switch mode power supply, power regulation is accomplished by applying pulse-width modulation to the switching transistors, in particular, the control of the on-time to off-time ratio of said transistors which operate at frequencies up to hundreds of kilo-hertz. The output voltage depends on the duty cycle of said control pulses and the input voltage, and it is essentially load independent. Moreover, changing the switching pulse width accordingly results in constant output voltage even when the input voltage varies.
A plurality of power converter topologies has been devised to address the different design issues such as power level, output voltage and input-output isolation. Flyback converters, for instance, require relatively large transformer cores and switching transistors, and they are suitable for applications which require low part count and low power levels. In the popular forward topology, energy is supplied to the output capacitor while the switching transistor is conducting, and it achieves significantly better transformer utilization than the flyback design. However, forward converters employing single switching transistor suffer from the same shortcoming as the flyback design, namely the voltage across said transistor is inherently unconstrained. This results in higher voltage rating requirement for the switching transistor, and large voltage transients which must be clamped by snubber circuit or additional reset winding. The undesirable power loss in said snubber circuit results in lower power conversion efficiency.
As illustrated in FIG. 1, a two-switch forward converter typically employs two switching transistors 406 & 407 on the primary side of transformer 102. Magnetizing current builds up during the conducting periods. When said transistors turn off and interrupt the current path, the magnetizing inductance acts as a voltage source. Reverse voltage from this inductive source forward biases and turns on the two diodes 403 & 404 to maintain current flow. In sufficient time, the magnetizing inductance is depleted by this voltage until the stored energy is returned to input source 401. Its clamped transformer voltage operation, with a maximum duty cycle of 50%, allows easy reset of transformer core. Thus, the voltage across said switching transistors is constrained to the input voltage and this allows lower-voltage and less expensive switching transistors to be used.
The conventional two-switch forward configuration has one known disadvantage, namely an isolated driver circuit 101 is required to couple pulses from controller 402 to transistor 407. Said drive circuit is commonly implemented with additional driver transformer or active semiconductor isolation device, typically together with external components, at the expense of increased bill-of-material cost, power consumption, part count, and overall circuit board estate.
Accordingly, there is an imperative need for innovative gate drive circuit designs which could meet the increasing demand for high-efficiency, low-cost and compact switch mode power supplies. The power converter of the present invention satisfies the need. Other advantages of this invention are apparent with reference to the detailed description provided herewith.